The problem: oil doesn't mix with water
Blood is mostly water. A water-loving hormone is happy there: an insulin or epinephrine molecule simply dissolves and floats along as a circulating hormone. But a fat-loving steroid or thyroid hormone would clump and fall out of solution — like oil in a glass of water. It needs a ride.
The ride is a carrier protein: a blood protein with a pocket shaped to hold the hormone. Bound to its carrier, a fat-loving hormone dissolves perfectly well in plasma and travels safely. At any instant, then, a lipophilic hormone exists in two pools — the tiny part floating loose, the free hormone, and the large part riding a carrier, the bound hormone.
Only the free fraction works
This is the single most important rule of hormone transport: only [[free-hormone|free hormone]] can leave the blood, enter cells, and act. A molecule tucked into a carrier's pocket is shielded — it cannot reach a receptor while it is bound. The bound pool is held in reserve. The free and bound pools sit in a balance: when free hormone is used up at the tissues, bound hormone is released to top it up, and when free rises, more gets captured.
Meet the carriers: SHBG, CBG, TBG
Some carriers are specific. SHBG (sex hormone-binding globulin) grips sex steroids — testosterone and estradiol. CBG (corticosteroid-binding globulin) carries cortisol. Thyroxine-binding globulin (TBG) carries T4 and T3. General-purpose carriers like albumin also pitch in, holding many hormones loosely.
Why care about carrier levels? Because changing the carrier changes the math of total versus free. If something raises SHBG — pregnancy and estrogen do — more testosterone gets bound, so a total testosterone reading can look normal while the free (active) amount has dropped. The body usually readjusts to defend the free level, but in the meantime a lab number can mislead unless you measure or estimate the free fraction.
Example: total cortisol vs free cortisol
Normal: total = 100 units, bound to CBG = 95, FREE = 5 (the 5 is what acts)
Estrogen up: CBG rises -> more cortisol bound
total measured = 150 units (looks high!)
but FREE still ~ 5 -> tissues feel the SAME cortisol
Lesson: a high TOTAL can hide an unchanged FREE. Always ask which one the test reports.